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MC68HC05B4 Datasheet, PDF (58/298 Pages) Motorola, Inc – High-density Complementary Metal Oxide Semiconductor (HCMOS) Microcomputer Unit | |||
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4.6
Other port considerations
All output ports can emulate âopen-drainâ outputs. This is achieved by writing a zero to the relevant
output port latch. By toggling the corresponding data direction bit, the port pin will either be an
output zero or tri-state (an input). This is shown diagrammatically in Figure 4-3.
When using a port pin as an âopen-drainâ output, certain precautions must be taken in the user
software. If a read-modify-write instruction is used on a port where the âopen-drainâ is assigned and
4
the pin at this time is programmed as an input, it will read it as a âoneâ. The read-modify-write
instruction will then write this âoneâ into the output data latch on the next cycle. This would cause
the âopen-drainâ pin not to output a âzeroâ when desired.
Note: âOpen-drainâ outputs should not be pulled above VDD.
Read buffer output
(a)
A
Y
Data direction register bit DDRn
DDRn A
Y
1
0
0
1
1
1

 Normal operation â tri state
0
0 tri state 
0
1 tri state 
(b)
1
0
low 
1
1
â

 âOpen-drainâ
0
0
high 
0
1
high 
VDD
VDD
Px0
âOpen-drainâ output
(c)
DDRx, bit 0 = 0
Portx, bit 0 = 0
DDRx, bit 0 = 0
Portx, bit 0 = 0
Figure 4-3 Port logic levels
MOTOROLA
4-6
INPUT/OUTPUT PORTS
TPG
MC68HC05B6
Rev. 4
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